[Payments] Add PaymentRequest checkout funnel UKMs.

tools/metrics/ukm/ukm.xml

 <!--
 Copyright 2017 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome UKM metrics
 along with a detailed description for each metric.
 -->
 
 <ukm-configuration>
 
+<event name="Autofill.CardUploadDecision">
+  <owner>sebsg@chromium.org</owner>
+  <metric name="UploadDecision">
+    <summary>
+      Whether the upload was proposed to the user or the reason why it was not.
+    </summary>
+  </metric>
+</event>
+
 <event name="PageLoad">
   <owner>bmcquade@chromium.org</owner>
   <summary>
     Recorded when Pageload observer events fire.
   </summary>
   <metric name="DocumentTiming.NavigationToDOMContentLoadedEventFired">
     <summary>
       Time from navigation to dom content loaded in ms.
     </summary>
   </metric>
   <metric name="DocumentTiming.NavigationToLoadEventFired"/>
   <metric name="Experimental.PaintTiming.NavigationToFirstMeaningfulPaint"/>
   <metric name="Navigation.PageTransition"/>
   <metric name="Net.EffectiveConnectionType.OnNavigationStart"/>
   <metric name="Net.ErrorCode.OnFailedProvisionalLoad"/>
   <metric name="PageTiming.ForegroundDuration"/>
   <metric name="PageTiming.NavigationToFailedProvisionalLoad"/>
   <metric name="PaintTiming.NavigationToFirstContentfulPaint"/>
   <metric name="ParseTiming.NavigationToParseStart">
     <summary>
       Time from navigation to parse start in ms.
     </summary>
   </metric>
 </event>
 
-<event name="Autofill.CardUploadDecision">
+<event name="PaymentRequest.CheckoutEvents">
   <owner>sebsg@chromium.org</owner>
-  <metric name="UploadDecision"/>
+  <metric name="CompletionsStatus">
+    <summary>
+      How the Payment Request ended. Values defined in the CompletionStatus enum
+      of components/payments/core/journey_logger.h.
+    </summary>
+  </metric>
+  <metric name="Events">
+    <summary>
+      Bitfield representing the events that occurred during the Payment Request.
+      Values defined in the Event enum of
+      components/payments/core/journey_logger.h.
+    </summary>
+  </metric>
 </event>
 
 <event name="Translate">
   <owner>hamelphi@chromium.org</owner>
   <summary>
     Metrics related to a Translate event. These metrics are described in
     TranslateEventProto.
   </summary>
   <metric name="AcceptCount"/>
   <metric name="Country"/>
   <metric name="DeclineCount"/>
   <metric name="EventType"/>
   <metric name="IgnoreCount"/>
   <metric name="RankerResponse"/>
   <metric name="RankerVersion"/>
   <metric name="SourceLanguage"/>
   <metric name="TargetLanguage"/>
 </event>
 
 </ukm-configuration>